In the field of image forming apparatuses, a technique for inputting an output voltage from a light-receiving sensor to an A/D converter for digitization and using the resultant for various types of control has been done.
When a voltage output from the light-receiving sensor (an output voltage) is to be input to the A/D converter through a predetermined path, it is desirable for accurate and efficient detection of the amount of light received by the light-receiving sensor so that a voltage input to the A/D converter (an input voltage) falls within an allowable range specific to the A/D converter and that the domain of the input voltage extends over the entire allowable range (i.e., the entire allowable range be effectively used).
However, it is difficult to configure the light-receiving sensor or a circuit peripheral thereto such that the domain of the input voltage extends over the entire allowable range of the A/D converter (i.e., the maximum value of the input voltage barely falls within the allowable range).
For example, voltages output by light-receiving sensors vary due to various factors such as individual differences among the light-receiving sensors themselves and individual differences among members, each of which is in the path of light emitted from a light-emitting portion to the corresponding light-receiving sensor. Accordingly, if the maximum output of the light-receiving sensor, the circuit constant of the peripheral circuit, and the like are set in consideration of such variation, the maximum value of the input voltage becomes considerably lower on average than the upper end of the allowable range. This makes it impossible to effectively use the entire allowable range of the A/D converter. On the other hand, if the light-receiving sensor or peripheral circuit is configured without taking such variation into consideration, the input voltage may fall outside the allowable range. In this case, part of the input voltage outside the allowable range cannot be used for light amount measurement.
The input range is likely to fall outside the allowable range not only due to the problem of variation but also due to circumstances where the maximum output of the light-receiving sensor needs to be increased or circumstances where the upper end of the allowable range of the A/D converter needs to be lowered. In this case as well, part of the input voltage cannot be effectively used for light amount measurement.
Thus, there is a need in the art for a light amount measuring device, image forming apparatus, and toner density measuring apparatus, capable of converting an output voltage from a light-receiving sensor appropriately and keeping the domain of an input voltage input to an A/D converter within the allowable range of the A/D converter easily.